The present invention relates to a luminescent device. In particular, it relates to a luminescent device which has two or more light-emitting regions. It also relates to a liquid crystal display device incorporating such a luminescent device.
One well-known luminescent device is an electroluminescent device. An electroluminescent (xe2x80x9cEL devicesxe2x80x9d) generates light as a result of electronxe2x80x94hole recombination. An EL device typically has a multilayer structure, in which a light-emitting layer is confined between an anode layer and a cathode layer. The emitter layer may be either an organic material or an inorganic material. Charge carrier recombination occurs in the emitter layer, and photons are generated. It is possible to vary the wavelength of light emitted by an EL device by using different materials for the emitter layer or by applying different drive conditions to the emitter layer, and it is also possible to manufacture an EL device that emits white light.
Many organic emitting materials have relatively broad emission and absorption spectra. If single colour emission, or a narrow colour range, is required, it is possible to use colour filters as disclosed in J. Kido et al, xe2x80x9cSciencexe2x80x9d Vol 267, page 1332 (1995). An alternative approach to obtaining a narrow wavelength range is to use cavity effects to narrow the emission spectrum, as described A. Dodabalapur et al, xe2x80x9cJournal of Applied Physicsxe2x80x9d Vol 80, page 6954 (1996). Although the output wavelength can be narrowed using cavity effects, this approach has the disadvantage that the output becomes very directional and this is undesirable in a display intended to be viewed from a wide range of angles.
In many applications it is desirable to provide a full colour display. One possible way to achieve a full colour EL device involves dividing each pixel into three sub-pixels, with the three sub-pixels lying side by side. One of the sub-pixels emits red light, one green light and one blue light. A device of this type is disclosed in U.S. Pat. No. 5,294,869. One disadvantage of this known device is that each colour is emitted from only one third of the total active area of the device, so that the intensity of the device is low.
An alternative approach to providing a full colour EL device consists of stacking two or more EL devices above one another. Devices of this type are disclosed in P. E. Burrows et al, xe2x80x9cApplied Physics Lettersxe2x80x9d Vol 69, No. 20, Nov. 11, 1996, pages 2959-2961, and in S. R. Forrest et al, xe2x80x9cSynthetic Metalsxe2x80x9d Vol. 91, pages 9-13 (1997).
A stacked, three-layer EL device is shown schematically in FIG. 1. This consists of a red EL element 1 disposed over a green EL element 2 which in turn is disposed over a blue EL element 3. Each EL element comprises a cathode layer 4B, 4G, 4R, an emitter layer 5B, 5G, 5R, and an anode layer 6B, 6G, 6R. Although FIG. 1 shows the EL elements as being separate from one another, in practice they would be stacked with an insulating layer separating each anode-cathode interface.
In the stacked EL device shown in FIG. 1, emission of light of each colour occurs over the entire active cross-sectional area of the device, so that the intensity of the device is improved compared to the device described above which uses laterally divided sub-pixels. However, the EL device shown in FIG. 1 has the disadvantage that light emitted by the red EL element must pass through the other two elements before it is emitted from the device, and that light emitted from the green EL device must pass through the blue EL device. This is a particular problem if organic materials are used to farm the emitter layers in the EL elements, since organic emitting materials generally have relatively broad emission and absorption spectra.
Forrest et al have attempted to address the problem of light emitted in one EL element being absorbed in a subsequent EL element. They have made use of the Stokes effect which provides a shift between the peak emission wavelength and the peak absorption wavelength.
The Stokes shift is illustrated in FIG. 2, which shows the emission and absorption spectra for the three EL elements of the EL device of FIG. 1. The letter xe2x80x9caxe2x80x9d indicates the absorption spectra, and the letter xe2x80x9cexe2x80x9d indicates the emission spectra. The Stokes shift appears as a shift between the absorption spectrum and the emission spectrum for an EL emitter layer. Forrest et al have chosen materials which have large Stokes shifts so as to minimise the absorption of radiation emitted by one EL element in other EL elements.
The devices proposed by Forrest at al have the following disadvantages. Firstly, the choice of materials for the emitter layers of the EL elements is restricted, owing to the need to use only materials with a large Stokes shift. Moreover, Forrest et al are constrained to use the particular order of the red, green and blue EL elements shown in FIG. 1, so that the red light (with a low energy) subsequently passes through emitter layers having a higher band gap. However, even if the red light is not absorbed across the band gap of the emitter layers in the green and blue EL elements, sole absorption of the red light will inevitably occur as it passes through the blue and green EL elements. The red EL element currently has the lowest intensity of the red, green and blue EL elements. It would thus be preferable to put the red EL element to the front so that the red light did not have to pass through the green and blue EL elements, rather than place it at the back as required by Forrest et al.
A further disadvantage with the prior art is that the EL devices will emit light in both the forward direction (as shown in FIG. 1) and in the backward direction. It would be desirable to utilise the light emitted in the backward direction, as well as the light emitted in the forwards direction, so as to increase the intensity of the device. It is possible to provide a mirror (not shown) above the red EL element of FIG. 1 to reflect the light emitted in the backward direction back towards the blue EL element 3. However, light emitted in the backwards direction by the green or blue EL elements will have to pass through the red EL element twice, once before it reaches the mirror and once after it has been reflected, so that significant absorption will occur. Thus, even if a mirror is provided much of the light emitted in the backward direction will be lost.
In an EL device having an organic emitting layer, the emitting layer is usually evaporated, or spun-down. This will produce an amorphous emitting layer, which emits light having no polarisation. In many applications, it would be desirable to produce an organic EL device that emits polarised light.
One known approach to providing an organic EL device that emits light having some degree of polarisation is to deposit the organic emitting layer with some degree of orientation. This can be done by techniques such as Langmuir-Blodgett deposition, mechanically deforming an organic emitting layer, or rubbing a pure conjugatedxe2x80x94polymer emitter layer. An alternative technique is to deposit a polymer layer on a highly aligned orientation layer such as polytetrafluoroethylene or polyimide, or by stretching a polymer layer. A further known technique is disclosed by Weder et al in xe2x80x9cAdvanced Materialsxe2x80x9d Vol. 9, page 1035 (1997), in which they disclose the tensile deformation of a guest-host system, so that the guest molecules adopt the orientation of the host.
An alternative approach to providing an organic emitting layer that emits light having some degree of polarisation is the cross-linking of polymeric materials using polarised UV light. This method eliminates the mechanical rubbing step, which is desirable since rubbing may introduce charge, inhomogeneities and dirt into the organic layer. M. Hasegawa et al, xe2x80x9cJ Photopolym Sci Technolxe2x80x9d Vol. 8, page 241 (1995), M. Schadt et al, xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d Vol. 31, page 2155 (1992) and M. Schadt et al, xe2x80x9cNaturexe2x80x9d Vol. 381, Page 212 (1996) disclose studies on cross-linking by polarised UV light. WO9707653A discloses an electroluminescent lamp consisting of a combination of an electroluminescent material to irradiate light and a reflecting polarisation multi-layered optical film so as to be able to pass a polarised light therethrough.
A first aspect of the present invention provides a luminescent device having first and second light-emitting regions, wherein the first light-emitting region emits, in use, a first polarised light and the second light-emitting region emits, in use, a second polarised light different from the first polarised light. A luminescent device according to this aspect of the Invention can be used to produce a patterned emitter, that emits polarised light in which the nature of the emitted light varies over the area of the device. Alternatively, a device according to this aspect of the invention can be embodied as a stacked device, to overcome the problems with the prior art stacked EL devices outlined above.
The first light-emitting region may emit, in use, light having a first polarisation and the second light-emitting region may emit, in use, light having a second polarisation different from the first polarisation. The first and second light-emitting regions may each emit, in use, plane polarised light, and the plane of polarisation of light emitted by the first light-emitting region may be different from the plane of polarisation of light emitted by the second light-emitting region. If the two light-emitting regions pan be controlled independently, it is possible to vary the polarisation of light emitted by the device.
The plane of polarisation of light emitted by the first light-emitting region may be at an angle of substantially 90xc2x0 to the plane of polarisation of light emitted by the is second light-emitting region. If the first and second light-emitting regions are stocked so that light from the first light-emitting region must pans through the second light-emitting region, this difference in the polarisation direction of the two light-emitting regions means that light emitted by the first light-emitting region will not be significantly absorbed in the second light-emitting region, so that light losses in a stacked EL device are reduced.
The first and second light-emitting regions may be disposed side by side. Alternatively, the first light-emitting region may be disposed over the second light-emitting region.
The device may further include a third light-emitting region, with the first light-emitting region disposed over the second light-emitting region, and the second light-emitting region being disposed over the third light-emitting region. If the three light-emitting regions are independently controllable and emit light of different wavelengths to one another, a full-colour light-emitting device can be obtained.
The third light-emitting region may emit polarised light having a third polarisation which is different from at least one of the first and second polarisations. The first, second and third light-emitting regions may emit plane-polarised light, with the plane of polarisation of the light emitted by the second light-emitting region being at substantially 60xc2x0 to the plane of polarisation of the light emitted by the first light-emitting region, and the plane of palarisation of light emitted by the third light-emitting region may be at substantially 120xc2x0 to the plane of polarisation of light emitted by the first light-emitting region. This will again reduce absorption of light emitted by the first light-emitting region as it passes through the second and third light-emitting regions, and will also reduce the absorption of light emitted by the second light-emitting region as it passes through the third light-emitting region.
The first light-emitting region may emit, in use, polarised light having a first wavelength and the second light-emitting region may emit, in use, polarised light having a second wavelength different from the first wavelength. If the two light-emitting regions are controllable independently, it is then possible to vary the wavelength of light emitted by the devices
The first and second light-emitting regions may each emit, in use, plane-polarised light. The plane of polarisation of light emitted by the first light-emitting region may be substantially parallel to the plane of polarisation of light emitted by the second light-emitting region. Alternatively, the first and second light-emitting regions may each emit, in use, circularly-polarised light, and the first and second light-emitting regions may each emit, in use, circularly-polarised light. The sense of palarisation of light emitted by the first light-emitting region may be the same as the sense of polarisation of light emitted by the second light-emitting region.
The device may be an electro-luminescent device.
A second aspect of the present invention provides a luminescent device having first and second light-emitting regions, wherein the emitter molecules in the first light-emitting region are aligned substantially in a first direction and the emitter molecules in the second light-emitting region are aligned substantially in a second direction, the second direction being different from the first direction. The first light-emitting region may emit, in use, light having a first polarisation and the second light-emitting region may emit, in use, light having a second polarisation different from the first polarisation. The polarisation of the emitted light arises from the alignment of the emitter molecules, and the difference in polarisation between light emitted from the first and second light-emitting regions is due to the different alignment directions in the two light-emitting regions. Alternatively, the first light-emitting region may emit, in use, light having a different wavelength to light emitted, in use, by the second light-emitting region.
A third aspect of the present invention includes a layer of liquid crystal material disposed over a backlight; wherein the backlight includes a luminescent layer, a first portion of the luminescent layer emitting, in use, polarised light at a first wavelength and a second portion of the luminescent layer emitting, in use, polarised light at a second wavelength.
The device may further include a first colour filter for transmitting light of the first wavelength and a second colour filter for transmitting light of the second wavelength, the first and second colour filters each being aligned with a respective one of the first and second portions of the luminescent layer.
The first and second portions of the luminescent layer may emit plane-polarised light. The plane of polarisation of light emitted by the first portion of the luminescent layer may be substantially parallel to the plane of polarisation of light emitted from the second portion of the luminescent layer. Alternatively, the first and second portions of the luminescent layer may emit circularly polarised light.
The luminescent layer may be an electroluminescent layer.
A fourth aspect of the present invention provides a method of manufacturing an emitter region for an electroluminescent device, the method including the steps of: evaporating an alignment layer over a substrate, the direction of evaporation being oblique to the substrate; and evaporating emissive molecules onto the alignment is layer so as to form the emitter layer, the direction of evaporation of the emissive molecules being substantially perpendicular to the substrate.
The orientation of the molecules in the emitter layer will be controlled by the alignment direction of the alignment layer, and this will depend on the angle between the evaporation direction and the substrate when the alignment layer is evaporated. This method can be used to manufacture a stacked EL device having two (or more) emitter layers with the emitter molecules in one emitter layer being aligned in a different direction from the emitter molecules in the other emitter layer, by growing each emitter layer over an obliquely deposited alignment layer.